STUDIECENTRUM VOOR KERNENERGIECENTRE D’ETUDE DE L’ENERGIE NUCLEAIRE

www.sckcen.beinfo@sckcen.be

2011_10Kbe1

Age constraints for pedological and hydrological processes in natural analogues of earth covers for waste disposal: case study from a sediment-soil sequence in Dessel,

Northern BelgiumKoen Beerten*,1, Koen Deforce2, Dirk Mallants1

1 SCK•CEN, Performance Assessments, Boeretang 200, 2400 Mol, Belgium - 2 Flemish Heritage Institute, Koning Albert II-laan 19 bus 5, 1210 Brussels, Belgium - * Corresponding author: kbeerten@sckcen.be

BackgroundProper assessement of the long-term evolution of earth covers for near-surface waste disposal demands knowledge on the rate of processes that may act upon such covers. We present age control for pedological and hydrological processes observed in a suitable natural analogue for engineered covers (sediment-soil sequence) observed in the Campine area, Northern Belgium. The following dating methods were used: OSL dating, C-14 dating, palynology, historical archives and stable Pb profiles.

Sediment-soil sequenceProfiles ZP1, ZP2 and ZP3 show a truncated podzol (1-5 ka BP ?) that developed in cover sands (~17 ka BP) (unit 1) which are overlain by a sequence of drift sand units (~300-600 ka BP) (units 2 to 5). The drift sands show phases of landscape stability (O2-horizons) and finally the formation of a micropodzol in the top of unit 5. The drift sands are overlain by a recently remobilized sand unit (unit 6).

References(i) Murray, A.S., Wintle, A.G., 2003. The single aliquot regenerative dose protocol: potential for improvements in reliability. Radiation Measurements 37, 377-381.(ii) Deforce, K., 2011. Pollen analysis of successive soil horizons from a dune profile from Dessel (N-Belgium). RNO.VIOE.2011-001. Vlaams Instituut voor het Onroerend Erfgoed. Brussel.(iii) Koster, A., 2009. The “European Aeolian Sand Belt”: Geoconservation of Drift Sand Landscapes. Geoheritage 1, 93-110.(iv) van Mourik, J.M., Nierop, K.G.J., Vandenberghe, D.A.G., 2010. Radiocarbon and OSL dating based chronology of a polycyclic driftsand sequence at Weerterbergen (SE Netherlands). CATENA 80, 170-181.(v) Jacques, D., Leterme, B., Beerten, K., Schneider, S., Finke, P., Mallants, D., 2010. Long-term evolution of the multi-layer cover. Project near-surface disposal of category A waste at Dessel V1. NIROND-TR 2010-03E. (vi) Seuntjens, P., 2000. Reactive solute transport in heterogeneous porous media – Cadmium leaching in acid soils. PhD thesis. Antwerp University, Belgium.(vii) Shotyk, W., Weiss, D., Appleby, P.G., Cheburkin, A.K., Frei, R., Gloor, M., Kramers, J.D., Reese, S., Van Der Knaap, W.O., 1998. History of atmospheric lead deposition since 12370 C-14 yr BP from a Peat Bog, Jura Mountains, Switzerland. Science 281, 1635-1640.

4

4/O2

3/O2 3/C

2/O2

2/C

5

Internal stratification drift sands

340 ± 40 yrs

360 ± 40 yrs

400 ± 40 yrs

510 ± 50 yrs

16.9 ± 1.2 kyrs 1/C

430 ± 50 yrs

2

3

4

5

ZP2

1/E 1/Bh1

1/Bh2

1/BC

5

4

3

2

ZP1

5/C

5/E

6/C

5/A1

6/O1

ZP3

10 cm

40 km

Layer Soil horizon Interpretation

6 O1 LitterC Recent sand remobilization

5 (drift sand)A1 MicropodzolEC Unaltered drift sand

4 (drift sand) O2 Digested litterC Unaltered drift sand

3 (drift sand) O2 Digested litterC Unaltered drift sand

2 (drift sand) O2 Digested litterC Unaltered drift sand

1 (cover sand)

EPodzolBh1

Bh2BC and C Unaltered cover sand

Optically stimulated luminescence (OSL) dating of quartzThe single-aliquot regenerative dose protocol was performed (240°C preheat and optical wash) using the RISOE TL/OSL-DA-20C/D Reader for dose determination(i). Several aliquots had to be rejected because of poor recycling and/or bad recuperation properties. Dose rates were measured using high-resolution gamma-ray spectrometry in the laboratory. The U-238 decay chain represents less than 20% of the total dose rate. The results indicate that drift sand deposition occurred between ~300 years and ~600 years ago (AD 1400-1700).

Sample Unit Depth

(cm)Moisture

content (%)U-238

(Bq/kg)Ra-226 (Bq/kg)

Ra-228 (Bq/kg)

K-40 (Bq/kg)

Dose rate (Gy/ka)

De(Gy)

No. aliquots

Age (ka BP)

BP2-OSL1 5 35 4 ± 2 < 11 6.0 ± 0.4 3.5 ± 0.5 126 ± 6 0.77 ± 0.03 0.26 ± 0.03 13 0.34 ± 0.04

BP2-OSL2 5 50 4 ± 2 < 8 5.3 ± 0.4 3.7 ± 0.4 121 ± 5 0.73 ± 0.03 0.26 ± 0.03 17 0.36 ± 0.04

BP2-OSL3 4 85 4 ± 2 < 11 5.2 ± 0.4 3.7 ± 0.5 121 ± 6 0.73 ± 0.03 0.29 ± 0.02 15 0.40 ± 0.04

BP2-OSL4 3 105 4 ± 2 < 8 4.3 ± 0.3 3.5 ± 0.5 110 ± 5 0.67 ± 0.03 0.29 ± 0.03 17 0.43 ± 0.05

BP2-OSL5 2 125 6 ± 3 < 8 6.1 ± 0.4 4.3 ± 0.5 122 ± 6 0.74 ± 0.03 0.38 ± 0.03 14 0.51 ± 0.05

BP2-OSL6 1 150 11 ± 3 < 7 8.1 ± 0.4 5.7 ± 0.5 172 ± 6 0.89 ± 0.04 15.1 ± 1.0 23 16.9 ± 1.2

Pollen analysis and historical archivesHistorical sand drifting is known to have been caused by agricultural practices several centuries ago (plaggen, overgrazing, etc.) while such landscapes became stabilized by pine reforestation in the late 18th and 19th century(ii),(iii). Samples from units 2, 3 and 4 (profile ZP2) show the dominancy of heather pollen (Calluna) and the diagnostic presence of buckwheat pollen (Fagopyrum) suggesting that these drift sand units were deposited in the timeframe AD 1400-1750. The dominancy of pine pollen (Pinus) in the uppermost micropodzol (unit 5/A1) suggests that it developed after dune stabilization and pine reforestation during the last 250 years.

UnitDepth (cm)

Pinus(%)

Calluna(%)

Fago -pyrum (%)

Rest AP (%)

Rest NAP (%)

5/A1 5 36.7 8 0 19 36.34/O2 60 1.7 37.7 0.4 18 42.2

3 95 1.1 57.4 0 22.5 192/O2 110 0.4 47.7 0.9 23.5 27.5

2 130 1.7 46.4 0 34.9 17

Conclusions• Consistent age control for earth surface processes from different dating techniques• Episodes of landscape stability and soil formation consistent with literature(iii),(iv)

• Rate of processes Leaching of organic matter in podzol: from 1 cm in < 250 yrs to 25 cm in < 4000 (?) yrs Development of O2 horizons in < 100 yrs Migration of Pb down to 1 m in < 250 yrs

Stable Pb-profileThe concentration of Pb with depth in drift sands from profile ZP2 is thought to be the result of leaching during the last 110 years(vi). This is consistent with the inferred stabilization age of the dune (~300 yrs BP), and Pb records elsewhere in Europe(vii).

Radiocarbon dating of soil organic matterBulk C-14 ages from the podzol soil range between ~ 3 and 5 ka BP. However, C-14 ages from samples originating from the drift sands are much older than the OSL ages, suggesting that the bulk organic matter is mainly derived from reworked (aeolian, bioturbation or human) organic matter from the podzol. The best available estimate for the start and end of podzolisation would then be ~ 5 ka and ~ 1 ka BP respectively, which is consistent with podzol ages from a similar site in the Netherlands(iv). However, an older age for the start of podzolisation cannot be excluded(v).

100

80

60

40

20

0

0 2 4 6 8

ppm Pb profile ZP2

concentration (ppm)

dept

h (c

m)

Profile: unit

Depth

(cm)

Cal. C - 14

age BP

( ± 2 σ )ZP2 : 4 95 2145 ± 145

ZP2: 2/O2 110 1090 ± 90ZP1: 1/E 115 3740 ± 100ZP1: 1/E 130 2795 ± 50

ZP1: 1/Bh 145 5085 ± 205